US3234349A - Expendable cap for fuse cutouts - Google Patents

Description

Feb. 8, 1966 R. J. BRONIKOWSKI 3,234,349

EXPENDABLE CAP FOR FUSE CUTOUTS Filed July 50, 1963 INVENTOR. Fey/vam 'J BRO/V/KOM/JK/ W4 T Q United States Patent Ofilice 3,234,349 Patented F eb. 8, 1966 3,234,349 EXPENDABLE CAP FOR FUSE CUTOUTS Ray on Bron ko s 70 ama ack 1-, South Milwaukee, Wis. Filed July so, 1963, Ser. No. 298,792 6 Claims; (Cl'. 200-127 This application is a. continuation-in-part of my. application. Serial No. 86,739 tiled. February. 2, 1961, now abandoned.

This invention relates generally to fuse cutouts. and more particularly to an improvement in expendable caps therefor.

Contemporaryfuse cutouts utilize a fuse tube which is vented at the lower. end and. which is provided at: the other end with a rupturable enclosing member. Upon low current faults, single. venting of the fuse tube takes place. However, on. higher faults up through themaxie murn interrupting capacity of the. cutout, the enclosing member. ruptures as a result of internal fuse. tube pressures acting thereon. Thepressure. increase Within the fuse tube is broughtabout by virtue of the fact thatgases are evolved within the confines of 'thetube. at a. much fasterv rate than they are vented therefrom.

US. Patent 2,702,842 to Jepsen et a1. discloses one of the most commonly utilizeddesigns for these expendable caps and has provenrelatively satisfactory as witnessed by its commercial success. However, in attempting to raise the interrupting capacity of a fusecutout of the type described to approximately double the standardratings, I have found that contemporary expendablecaps limit the interrupting capability of the cutout to alevel approximately 25% below the desired ratings. This has been largely due to the fact that the high pressure gases within the fuse tube, upon blowwoif of thev expendable cap, tended to drift or be blown downwardly along. the fuse tube eventually bridging the tube contacts and causing fiashover of the tube. Also, these suddenly released gases often were blown or attracted to the grounded hangar bracket, and/o r-parts associated therewith, causing flashover betweenthe bracket and current carrying portions of the cutout (line to ground). In both cases, the operating performance of the cutout is seriously impaired'and burned or otherwisedamaged cutouts resulted. Due to the above effects, the effective current that a cutout could interrupt was approximately 25% below the desired interrupting rating of the device. 1

Ihave found that the patterns assumed by the gases escaping from a fuse tube, subsequent to blow off of the expendable cap, are the. main cause of this 2.5% reduction of the interrupting capability of a fuse cutout. In this connection I have designed an improved cap which precludes the formation of the deleterious gas patterns which cause reductions in cutout interrupting ability.

It is therefore an object of this invention to provide an improved expendable cap which is adaptable to present cutout structures.

Another object of this invention is to provide an improved expendable cap which will function in such a manner that the interrupting capability of the fuse cutout is not materially reduced as a result of the formation of gas patterns which are conductive to flashover.

Another object of the invention is to provide. an improved expendable cap which is economical to produce and which is dependable in operation.

A further object of the invention is to. provide an expendable cap which will blow off at values of internal tube pressure below 1000 p.s.i.

A still further object of the invention is to provide an improved expendablecap which will blow off at minimum current values of -90% of those at which contemporary caps blow.

Other objects and advantages of my invention will be apparent from the following description of the preferred embodiments of the invention taken in connection with the accompanying drawings in which:

FIG. 1 is a partially cut away view in elevation of a fuse cutout incorporating an improved expendable cap.

FIG. 2 is a cutaway. view in elevation of a portion of the fuseholder assembly and expendable cap of FIG. 1.

FIG. 3 is a partially cutaway view in elevation of the expendable capof the invention.

Referring to FIGURE 1, 10 indicates a fuse cutout generally which comprises a skirted insulating member 11 having an upper contact assembly 12 and lower bearing members 13 afiixed thereto. The upper contact assembly 12. includes. a pivotal latching member 14, resilient contacts.15.and terminal seat 16. Associated with the bearingmember 13 are lower resilient contacts 17 and a lower terminal member 18.

A fuseholder. assembly 19 is journalled in the bearings Band is normally adapted to electrically bridge the contacts 15 and.17. The assembly 19 includes a fuse tube 20 which is capable of evolving quantities of gases, upper fuse tube contact assembly 21, pull ring assembly 22 and hinged trunnion and contact means 23. The fuse tube contact assembly 21 includes a ferrule member 24 which is fixedly attached to one end of the fuse tube 20. and which. has a projecting conducting portion 25, the outer endof which is normally adapted to engage contacts 15. The upper portion of the ferrule member24 has external screw threads 26 thereon which serve as a seat for ex.- pendable enclosing means 27 which is pressure sensitive and whichwill be explained in greater detail hereinafter. A rupturable fuse link 2.8 is positioned within the fuse tube 20 and is inelectrical engagement with fuse tube contact assembly 21 and anchored at its other end to member 23.by means 29.

As can be seen, the fuseholder is normally in contact bridging position. When a fault is experienced; on the line the overcurrent will. serveto melt the fuse link and the resultant arc in the fuse tube will be extinguished as a result of the gases evolved from the fuse tube. Initially, these evolved gases will vent from the bottom of thefuse tube but as the rate of their evolvement exceeds the rate at which they are vented a pressure build up in the tube occurs. This pressure build up, if sufliciently high, will cause the expendable enclosing (cap) means to rupture thereby allowing gases to vent from both ends of the fuse tube. Subsequent to are extinction, as a result of the rupturing of the fuselink, relative movement between the fuse tube and trunnion means 23 occurs and the fuseholder assembly thereafter pivots aboutbearin g members; 13 and out of contact bridging position.

Referring now to FIGS. 2 and 3, the expendable cap is. indicated generally at 27 and comprises a portion 30:

having internal screw threads 30 which are adapted to engage the external screw threads 26 on ferrule member 24. The enclosing portion of the cap 27 includes a shoulder portion 31, an inwardly spaced annular rupturable (frangible) portion 32, a first annular projection 33 and a central raised portion 34, thereby forming a bottom wall on said cup-shaped cap 27 which increases in thickness progressively in a direction radially inward from said frangible portion 32.

The upper end of the fuse link 28 comprises a reduced fusible portion 35 and a bottom head 36 which is normally of a somewhat greater diameter than the bore of the ferrule member 24 and Which rests upon the upper extremity of the ferrule member. As can be seen the upper surface of the bottom head 36 abuts against the relatively flat inner surface of the cap 27. A fibre liner 37 surrounds the fuse link and fusible portion thereof and extends into the bore of the fuse tube 20 for a distance and has for its purpose the confining and extinguishing of low current arcs.

In operation, as gases are evolved from the interior of the fuse tube, a force is exerted by these gases on the underside of the button head and uniformly distributed to the under surface of the cap immediately above the button head. The portion 32 is the thinnest portion of the cap and is designed to rupture under a given internal pressure. Upon rupture 'of this section of the cap, it and the button head will blow out, thereby allowing gases to be vented from that end of the tube. A large portion of the force exerted by. the button head is transmitted to the first projection 33'and central raised portion 34 of the cap. This causes bowingfor bulging of the bottom wall portion of the cap which increases progressively in a radially inward direction and thereby causes the rupturable portion 32 of the cap to be put in tension. This tensile force on area 32 is in addition to the shear force exerted on this area with respect to the shoulder portion 31 of the cap. The particular configuration of the cap which results in rupture occurring due to tensile and shear forces enables a closer control of the rupturing point to be obtained as well as permitting rupture at a more optimum internal tube pressure than would otherwise be possible, if only shear forces effected rupture. Contemporary caps rupture at internal pressures in the range of 1700-2300 p.s.i. which is in accordance with the industry standards of 2000 p.s.i. plus or minus percent.

This invention basically involves an expendable cap in which the rupturable section of the cap is designed to rupture at static bursting pressures of 1000 p.s.i. or less. A fuse cutout having an expendable cap in accordance with the invention has a maximum interrupting capacity approximately on-third higher than the same fuse cutout with a conventional cap. Tests establish that a fuse cutout rated at 100 amperes having an expendable cap in accordance with the invention will repeatedly interrupt 17,000 amperes successfully, Whereas the same fuse cutout with a conventional cap cannot clear currents above 12,000 amperes repeatedly without flashover. If the pressure rises above 1000 p.s.i., gas patterns occur which are conducive to fuse tube flashover or line to ground flashover, both of which may cause considerable damage to the equipment to be protected, the fuse cutout and associated structures. Hence, if the static bursting pressure of the cap is not kept to values of 1000 p.s.i. or less,

Expendable caps in accordance with the invention which are designed to rupture (blow off) at internal tube pressures of 1000 p.s.i or less will provide double fuse tube venting in a shorter time after arcing has commenced and at minimum currents which are 80 to 90% of those encountered when conventional caps rupture.

The frangible portion 32 preferably has a minimum thickness which will prevent rupture thereof at pressures below approximately 600 p.s.i. Such minimum thickness of frangible section assures that the cap will not rupture in the current range below 1500 amperes wherein clearing of the arc is efiected more eificiently by venting at a single end of the tube.

In this manner, the effective interrupting capability of the cutout is not reduced by the sudden high pressure gases venting from the tube and setting up flashover gas patterns, since the pressure at which the gases are vented is below that at which these deleterious gas patterns form.

While one particular embodiment of the invention has been shown and described, it will be obvious to those skilled in the art that various changes and modifications can be made therefrom without departing from the invention and, therefore, it is intended for the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. In an expulsion fuse cutout, in combination, a fuse tube open at both ends, a metallic ferrule aflixed to one end of said fuse tube, a fusible element within said fuse tube, an expendable metallic cap closing said one end of said fuse tube and having a threaded side wall portion engaging said ferrule and a bottom wall portion extending above the opening in said one end of said fuse tube and-means joiningsaid sideand bottom Wall portions and being rupturable at internal tube. pressure from 600 to 1000 p.s.i. to separate said side and bottom wall portions and vent said one end of said fuse tube, whereby gas patterns which cause electrical flashover of said cutout are prevented and the interrupting capacity of said fuse tube is increased.

2. A generally cup-shaped expendable .cap adapted to be affixed to and close one end of a fuse tube open at both ends, said cap having a threaded continuous sidewall portion and a bottom wall portion adapted to extend above the opening in said one end of said fuse tube and means joining said side and bottom wall portions and being partible to separate said side and bottom wall portions at internal tube pressures from 600 to 1000 p.s.i. and at pressures below those which are accompanied by patterns of expelled gases conducive to flashover of said fuse tube.

'3. A generally cup-shaped expendable cap adapted to be affixed to and enclose one end of a fuse tube, the bottom wall of said cup-shaped cap having means defining an annular frangible section of such thickness and strength that it will rupture and the portion of said bottom wall surrounded by said frangible section will separate from the remainder of said cap when subjected to internal tube pressures from 600 to 1000 p.s.i. and below those pres sures which are accompanied by patterns of expelled gases conducive to flashover of said fuse tube. V

4. A generally cup-shaped expendable cap adapted to enclose one end of a fuse tube, the tubular sidewall portion of said cup-shaped cap being threaded to engage said one end of-said fuse tube and the bottom wall of said cupshaped cap having means defining an annular frangible section of such thickness and strength thatit will rupture and the portion of said bottom wall surrounded by said frangible section will separate from the remainder of said cap when subjected to internal tube pressures from 600 to 1000 p.s.i. and below those pressures which are accompanied by patterns of expelled gasesconducive to flashover of said fuse tube, the thickness of said bottom wall of said cap increasing progressively in a direction radially inward from'said annular frangible section.

5. In an expulsion fuse cutout, in combination, a fuse tube open atboth ends and having a material on its inner periphery adapted to evolve arc-extinguishing gas when subjected-to an electric arc, a metallic ferrule afiixed to one end of said fuse tube, a fusible element within said fuse tube having a metallic button head at one end disposed against said ferrule, a cup-shaped expendable metallie cap removably secured to said ferrule and closing said one end of said fuse tube, the bottom wall of said cupshaped cap being disposed above said button head and having means defining a frangible portion of such thickness and strength that it will rupture and separate from the remainder of said cap at internal pressures within said fuse tube between 600 and 1000 p.s.i. and which are below those pressures which produce patterns of expelled gases that fiashover said fuse cutout.

6. In an expulsion fuse cutout in accordance with claim 5 wherein said frangible portion is annular and the bottom Wall of said cap increases progressively in thickness References Cited by the Examiner UNITED STATES PATENTS 2/1955 Jepson 200-127 8/1958 Wood 200127 10 ROBERT K. SCHAEFER, Acting Primary Examiner.

BERNARD A. GILHEANY, Examiner.

Claims (1)

1. 2. A GENERALLY CUP-SHAPED EXPENDABLE CAP ADAPTED TO BE AFFIXED TO AND CLOSE ONE END OF A FUSE TUBE OPEN AT BOTH ENDS, SAID CAP HAVING A THREADED CONTINUOUS SIDEWALL PORTION AND A BOTTOM WALL PORTION ADAPTED TO EXTEND ABOVE THE OPENING IN SAID ONE END OF SAID FUSE TUBE AND MEANS JOINING SAID SIDE AND BOTTOM WALL PORTIONS AND BEING PARTIBLE TO SEPARATE SAID SIDE AND BOTTOM WALL PORTIONS AT INTERNAL TUBE PRESSURE FROM 600 TO 1000 P.S.I. AND AT PRESSURES BELOW THOSE WHICH ARE ACCOMPANIED BY PATTERNS OF EXPELLED GASES CONDUCIVE TO FLASHOVER OF SAID FUSE TUBE.

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